In the past in the manufacturing of integrated circuits, after the individual devices, such as the transistors, have been fabricated in the silicon substrate, they must be connected together to perform the desired circuit functions. This connection process is generally called "metalization", and is performed using a number of different photolithographic and deposition techniques.
One metalization process, which is called the "damascene" technique, starts with the placement of a first channel dielectric layer, which is typically an oxide layer, over the semiconductor devices. A first damascene step photoresist is then placed over the oxide layer and is photolithographically processed to form the pattern of the first channels. An anisotropic oxide etch is then used to etch out the channel oxide layer to form the first channel openings. The damascene step photoresist is stripped and an optional thin adhesion layer is deposited to coat the walls of the first channel opening to ensure good adhesion and electrical contact of subsequent layers to the underlying semiconductor devices. A barrier layer is then deposited on the adhesion layer improve the formation of subsequently deposited conductive material and to act as a barrier material to prevent diffusion of such conductive material into the oxide layer and the semiconductor devices (the combination of the adhesion and barrier material is collectively referred to as "barrier layer herein). It should be noted that some barrier materials also have good adhesion which is why the adhesion layer is optional. A "seed" layer is then deposited to act as a seed for additional conductive material to be deposited. A first conductive material is then deposited and subjected to a chemical-mechanical polishing process which removes the first conductive material above the first channel oxide layer and damascenes the first conductive material in the first channel openings to form the first channels.
For multiple layers of channels, another metalization process, which is called the "dual damascene" technique, is used in which the channels and vias are formed at the same time. In one example, the via formation step of the dual damascene process starts with the deposition of a thin stop nitride over the first channels and the first channel oxide layer. Subsequently, a separating oxide layer is deposited on the stop nitride. This is followed by deposition of a thin via nitride. Then a via step photoresist is used in a photolithographic process to designate round via areas over the first channels.
A nitride etch is then used to etch out the round via areas in the via nitride. The via step photoresist is then removed, or stripped. A second channel dielectric layer, which is typically an oxide layer, is then deposited over the via nitride and the exposed oxide in the via area of the via nitride. A second damascene step photoresist is placed over the second channel oxide layer and is photolithographically processed to form the pattern of the second channels. An anisotropic oxide etch is then used to etch the second channel oxide layer to form the second channel openings and, during the same etching process to etch the via areas down to the thin stop nitride layer above the first channels to form the via openings. The damascene photoresist is then removed, and a nitride etch process removes the nitride above the first channels in the via areas. An adhesion layer is then deposited to coat the via openings and the second channel openings. Next, a barrier layer is deposited on the adhesion layer. This is followed by a deposition of the second conductive material in the second channel openings and the via openings to form the second channel and the via. A second chemical mechanical polishing process leaves the two vertically separated, horizontally perpendicular channels connected by a cylindrical via.
The use of the damascene techniques eliminates metal etch and dielectric gap fill steps typically used in the metalization process. The elimination of metal etch steps is important as the semiconductor industry moves from aluminum to other metalization materials, such as copper, which are very difficult to etch.
One drawback of using copper is that copper diffuses rapidly through various materials. Unlike aluminum, copper also diffuses through dielectrics, such as oxide. When copper diffuses through dielectrics, it can cause damage to neighboring devices on the semiconductor substrate. To prevent diffusion, materials such as tantalum nitride (TaN), titanium nitride (TiN), or tungsten nitride (WN) are used as barrier materials for copper. A thin adhesion layer formed of an adhesion material, such as pure tantalum (Ta), titanium (Ti), or tungsten (W), is first deposited on the dielectrics or vias to ensure good adhesion and good electrical contact of the subsequently deposited barrier layers to underlying doped regions and/or conductive channels. Barrier layer stacks formed of adhesion/barrier materials such as tantalum/tantalum nitride (Ta/TaN), titanium/titanium nitride (Ti/TiN), and tungsten/tungsten nitride (W/WN) have been found to be useful as adhesion/barrier material combination for copper interconnects.
The "barrier effectiveness" of a barrier layer with respect to a conductive material is its ability to prevent diffusion of the conductive material. The barrier effectiveness of a barrier layer is determined in part by its thickness, including the thickness uniformity or density, and its quality, including the number and sizes of defects such as pinholes which form on deposition.
Barrier layers in a damascene process are traditionally deposited by sputter deposition. A problem associated with this deposition technique is that the nitrided barrier material is sputtered from the barrier metal target in a nitrogen atmosphere and is very difficult to control. The composition of the barrier metal/barrier material is very difficult to control and is susceptible to particle problems since two separate deposition steps, usually sputtering, are involved. In addition, the barrier metal target often becomes "poisoned" with nitrogen so it can not be used for forming the adhesion layers for subsequent semiconductors.
A solution, which would allow better control of the barrier metal/barrier material composition, eliminate particle problems, and avoiding target poisoning has long been sought, but has eluded those skilled in the art. As the semiconductor industry is moving from aluminum to copper and other type of materials with greater electrical conductivity and diffusiveness through dielectrics, it is becoming more pressing that a solution be found.